CMake: add -Wno-missing-braces to CXXFLAGS

This warning interferes with *correct* initialiser list usage
with std::array.

See:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=25137
ot_utility_ot: use std::array instead of home-brewed container
Merge pull request #103 from toxeus/useArray

ot_utility_ot: use std::array instead of home-brewed container